Transfer factor will be isolated from leucocytes of guinea pigs sensitive to 2,4-dinitrofluorobenzene, citraconic anhydride, tuberculin and coccidioidin. Attempts at complete separation and purification of transfer factor will be made. The materials will be compared by chemical, physical and biological tests. Passive transfer in vivo as well as the in vitro correlates of delayed hypersensitivity- lymphocytotoxicity, lymphocyte transformation and migration inhibition will be employed. The biological effects of dual sensitization will be studied.